Heating elements for vacuum furnaces



Nov, 3, 196% c. A. s. HELL HEATING ELEMENTS FOR VACUUM FURNACES Filed June 14, 1962 INVENTOR CHARLES A5. HILL ATTORNZ'V United States Patent HEATING ELEMENTS FOR VACUUM FURNACES Charles A. S. Hill, Southampton, Pa. Abar Corporation, 2424 Wyandotte Road, Willow Grove, Pa.) Filed June 14, 1962, Ser. No. 202,616 9 Claims. (Cl. 13 -25) This invention relates to resistance heating elements for vacuum and other furnaces. The necessity for heating and melting high temperature refractory metals, and specifically zirconium, columbium, tantalum, molybdenum, tungsten, titanium, rhenium, and

alloys thereof, which require heating in a vacuum not only for degassing the metal but to prevent undesired reaction has resulted in the use of vacuum furnaces.

Serious temperature limitations have been imposed by actual operating conditions on the heating elements heretofore available.

The maximum operating temperature for compacted sheathed type heating elementsis of the order of 1500" F. while brick lined furnaces can be operated up to about 2500 F. using nickel chromium elements. Improvementsin the wall construction of furnaces, and in the support of the sheathing elements, have permitted in creases in the temperatures attained up to about 3000 F. which is now regarded as the upper limit for economical, practical and extended operation.

Reaction of the metals being heated, or work charge, as well as the heating elements themselves have necessitated the use of vacuum furnaces from which the undesired reactive agents have been removed.

Failure of the resistance heating elements heretofore available has been due to a variety of causes including reaction to gases in the air or other detrimental reaction, sublimation of the elements, the failure due to high current density, failure because of reverse thermal stresses in heating up and cooling down, lack of strength at elevated temperatures so that sagging occurs, and failure of insulation.

The heating elements used in vacuum furnaces were originally of nickel-chromium alloys in ribbon and rod form supported upon ceramic insulators. Such element designs were limited to an element temperature of 1800 F. at an absolute pressure below one micron of mercury. Failure occurred from short circuiting of the elements from metallic deposits on the insulators as well as sublimation of chromium from the basic alloy. This limitation was overcome by the use of molybdenum, tungsten, and other refractory metal alloys in the form of ribbon, wire and rods. This raised the operating temperature of the furnace to about 3000 F. but the breakdown characteristics of the ceramic insulators imposed limitations on higher temperature capability.

Furnaces operating at temperatures above 3000 F. cannot successfully employ ceramic insulators because of thermal shock, electrical conductivity, oxidation of heating elements, loss of physical strength, and the activity of the ceramic material as a continuous source of gas at high vacuum, frequently referred to as outgassing.

In efforts to construct furnaces capable of operating up to about 3600 F., cylindrical sheet type of element construction was undertaken but such elements were subject to sagging and distortion attendant upon the nature of the support. Distortion also occurred because of dif-,

ferent and steep temperature gradients in the metal, with no suitable provisions for accommodating dimensional changes. The reverse mechanical stresses during each cycle resulted in accelerated fatigue with excessively short life of the heating elements.

The change in shape of the heating elements upon distortion also seriously limited the available heating space.

Sheet type heating elements of cylindrical shape have an interior temperature profile in which the temperature gradient is severe so that uniformity of heating of the work charge is virtually impossible to attain.

It is the principal object of the present invention to provide a resistance heating element for vacuum furnaces which will permit attaining higher temperatures than heretofore, which will have a longer useful life and which will be free from other difficulties heretofore encountered in this field.

It is a further object of the present invention to provide a resistance heating element for vacuum furnaces, which accommodates the elevated temperatures with negligible distortion, which has an improved current'input distribution for energization, and which has other improved qualities in use.

It is a further object of the present invention to provide a resistance heating element for vacuum furnaces which is simple in construction, sturdy, and free from difficulties in use.

It is a further object of the present invention to provide a resistance heating element closely approaching isothermal conditions in the interior thereof.

It is a further object of the present invention to provide a resistance heating element which is supported in a manner which decreases the likelihood of failure.

It is a further object of the present invention to provide a resistance heating element supported in a pendant relation and free from expansive restraint in all directions.

It is a further object of the present invention to provide a resistance heating element which in one embodiment can be assembled in condition for use without brazing, welding or other fusion operations. 1

It is a further object of the present invention to provide a resistance heating element of tungsten which is free from limitations inherent in the attempted fusion joining of this metal.

Other objects and advantageous features of the invention will be apparent from the description and claims.

The nature and characteristic features of the invention will be more readily understood from the following description taken in connection with the accompanying drawings forming part thereof, in which:

FIGURE 1 is a view in perspective of one preferred embodiment of the invention;

FIG. 2 is a vertical sectional view taken approximately on the line 22 of FIG. 1;

FIG. 3 is a view similar to FIG. 2 showing a modification of the structure of FIGS. 1 and 2;

FIG. 4 is a view in perspective of another preferred embodiment of the invention;

FIG. 5 is a plan 4; and

FIG. 6 is a wiring diagram forthe embodiment of the ious modifications and changes can be made in the struc- 'ture disclosed without departing fromthe spirit of the invention.

views.

Referring now more particularly to FIGS. 1, 2 and 6 of the drawings, four vertical posts 10, 11, 12 and 13,

are shown which are mounted in the interior of the vacuum furnace (not shown). The posts 10, 11, 12 and 13 Patented Nov. 3, 1964 view of the structure shown'in Like numerals refer to like parts throughout the several Each diagonally opposite pair of posts, such as the pair of posts and 12 are connected to one phase, and the other pair, such as the posts 11 and 13, are connected to another phase.

- The posts 10, 11, 12 and 13 serve as input terminals and for this purpose have shoulders and upwardly projecting portions 21 of reduced diameter. The posts 10, 11, 12 and 13 are of highly electrical conductive metal, such as copper, and are preferably intcriorly water cooled.

The resistance heating unit shown generally at 25 in FIGS. 1 and 2 includes two similar sections each of 21 single strip of sheet metal of block U-shape having a horizontal bottom wall portion 26 and integral therewith vertical side wall portions 27 with continuous side mar ginal edges 28.

As shown in FIGS. 1 and 2 the bottom wall portions 26 are slightly spaced, while as shown in FIG. 3 the bottom wall portions 26 are in contact and are preferably secured together by spot welding. The material from which the wall portions 26 and 27 is made must be such as will have suitable resistance to electric current flow so as to provide an adequate heat source and be resistant to reverse thermal stresses. It must be capable of withstanding heating to provide temperatures of 5000 F. or higher. The wall portions 26 and 27 must also be capable of relatively rapid cooling, subsequent to heating, without destructive effect. For this purpose tantalum and its alloys in sheet form has been found suitable although when subjected to excessive temperatures and under high vacuum it tends to sublime. For still higher temperatures, tungsten and tungsten alloys in sheet form has been found suitable and permits temperatures up to about 5500 F. Zirconium, eolumbium, molybdenum, rhenium, and their alloys, may also be used, in sheet form, where somewhat lower temperatures are desired.

In order to support the unit 25, spaced supporting arms 29 are provided, connected at their outer ends to metallic electrical conducting spacer blocks 30 and with their inner ends 31 turned outwardly and secured, as by spot welding, to header bars 24 extending the width of and secured to the vertical side wall portions 27 at the top of the outer faces thereof. The arms 29 are preferably made of the same metal as the contiguous side wall portions 27 so as to have the same temperature coclficient of expansion and thus reduce the likelihood of distortion and undue stressing upon expansion or contraction.

The centers of the ends 31 of the arms 29 are preferably at about quarter points measured along the top of the wall portions 27 to provide a better current distribution with respect to the side wall portions 27.

The blocks 30 have central openings 32 for engagement on the projecting portions 22 and for support by the shoulders 21.

The interior of the space above the bottom wallp ortions 26 and between the side wall portions 27 can be closed at the top in any desired manner such as by a top closure plate 33 capable of serving asa heating element and operating at the same temperature as thewall portions 26 and 27. The plate 33 has side supporting wings 34 extending from a pair of opposite side margins and these are preferably thicker to carry the current to the plate 33 without undesired resistance heating of the wings 34.

The supporting wings 34 have openings 35 for engagement on the projecting portions 22 and support by the arms 29 and blocks 30. The closure plate 33 and wings 34 are preferably made of the same metal as the unit 25 to equalize the expansion and contraction upon change of temperature.

The top closure plate 33 can have an opening 351 therein for insertion of any desired device for supporting the article to be heated.

Referring now more particularly to FIGS. 4 and 5 another preferred form of the invention is illustrated in which the heating unit 25a has the side wall portions 27a composed of closely spaced strips 36 of one of the metals previously mentioned, i.e. tantalum or tungsten, with the bottom part 26a having the strips interlaced in a basket weave. The top ends of the side wall portions 27a have inner connecting bars 37 of the same metal along the inner faces of the strips 36. The inner ends 31a of the arms 29 are elongated and the strips 36, the inner bars 37 and the inner ends 31a are secured together by bindings 38 of metallic wire of the same metal as the strips 36 or more ductile alloys of such metal. Electrically considcrcd, the interlacing of the strips 36 at the bottom providcs a structure which more closely resembles that of FIG. 3 in the connection at the intersection of the resistances provided by each of the side wall portions, 27.

In FIG. 6 the resistances R26 around the center correspond to the resistances of the side wall portion 27 with no common or interconnection for all four resistances. The structure of FIGS. 3, 4 and 5 would result in a common central interconnection.

In use, electric energy supplied to the posts 10 and 12 p in one phase and 11 and 13 in another phase from common conductors through the transformers is effective for heating the wall portions 26 and 27, and the top closure plate 33. As the heating occurs with expansion due to heating the side wall portions are relatively free from restraint horizontally considered and likewise are free from vertical restraint.

Even with the wall section 26 connected as in FIGS. 3, 4 and 5, there is still no restraint of undesirable nature, upon expansion or upon contraction with cooling.

The spacing of the marginal edges 28 providing open corners facilitates evacuation of the interior space within the heating unit, and reduces distortion on the lower part of the heating unit 25. The light loss at this location is minimized and is less than that normally to be expected for the width of the gap by the internal reflective bypass which provides a crossing of the spaces between the edges 28 with relatively low escape.

In the embodiment of the invention shown in FIGS. 4 and 5, and particularly if tantalum is employed, a self compensating action occurs in that if parts of the strips 36 are not in contact with the ends 31a or bars 37 at their upper ends when the heating occurs contact is made because of the dimensional changes.

In bothembodiments of the invention the direct heat transfer from the wall portion 27 to the supporting arms 29 is reduced because of the small cross sectional area for heat transfer.

The construction of heating element heretofore described has a uniformity of current distribution with improved resistance heating action. It further, by reason of the uniformity of heating has a substantially uniform temperature profile. Freedom from distortionboth upon heating and cooling is attained.

I claim: i

'1. A resistance heating element for furnaces composed of a high temperature refractory-metal and having a plurality of sections with horizontal bottom wall portions and opposite flat vertical side wall portions free from expansive restraint, and electrically conducting supporting members at the top of each of the side wall portions and extending horizontally outwardly therefrom and supporting said side wall portions free from zontal restraint.

2. A heating element as defined in claim 1 in which said supporting members include spaced horizontal arms connected at their outer ends and having their inner ends turned outwardly and disposed in the same plane,

3. A heating element as defined in claim 1 in which opposite pairs of electrical conducting posts are provided in supporting engagement respectively with the supporting members.

4. heating element as defined in claim 1 in which each of said sections includes a continuous strip of sheet metal.

vertical and hori- 5. A heating element as defined in claim 1 in which each of said sections is composed of a continuous strip of sheet metal and the bottom wall portions are in spaced relation.

6. A heating element as defined in claim 1 in which each of said sections includes a continuous strip of sheet metal and the bottom wall portions are in connected rela tion.

7. A heating element as defined in claim 1 in which each of said sections is composed of a plurality of strips of sheet metal in-edge to edge relation.

8. A heating element as defined in claim 1 in which a closure member is provided having opposite wing portions for support by an opposite pair of said supporting 'members.

9. A heating element as defined in claim 1 in which a closure member is provided for closing the end of said scctionopposite the bottom wall portions, said closure member constituting a resistance heating clement.

References Cited in the file of this patent UNITED STATES PATENTS 1,945,393 Chapman Jan. 30, 1934 2,223,617 Johnston Dec. 3, 1940 2,694,740 Lang Nov. 16, 1954 2,945,756 Ballantine July 19,1960 3,033,547 Baker et a1. May 8, 1962 3,057,936 Hill Oct. 9, 1962 

1. A RESISTANCE HEATING ELEMENT FOR FURNACE COMPOSED OF A HIGH TEMPERATURE REFRACTORY METAL AND HAVING A PLURALITY OF SECTIONS WITH HORIZONTAL BOTTOM WALL PORTIONS AND OPPOSITE FLAT VERTICAL SIDE WALL PORTIONS FREE FROM EXPANSIVE RESTRAINT, AND ELECTRICALLY CONDUCTING SUPPORTING MEMBERS AT THE TOP OF EACH OF THE SIDE WALL PORTIONS AND EXTENDING HORIZONTALLY OUTWARDLY THEREFROM AND SUPPORTING SAID SIDE WALL PORTION FREE FROM VERTICAL AND HORIZONTAL RESTRAINT. 